Ancient Bones Spark Fresh Debate over First Humans in the Americas

Who were the first Americans and when and how did they get here? For decades archaeologists thought they knew the answers to these questions. Based on the available evidence, it seemed that big game hunters from Asia known as the Clovis people were the first to blaze that trail, trekking across the now submerged land mass of Beringia to enter the New World around 13,000 years ago.

But starting in the early 2000s, signs of an earlier human presence in the Americas started to crop up, eroding support for the so-called Clovis First model. A new understanding of how people finally conquered the New World began to take shape: Homo sapiens arrived by boat by at least 15,000 years ago, following the western coast of the Americas.

Now the scientists behind a new discovery are looking to rewrite the story of human colonization of the Americas once again—and in a far more radical fashion. In a paper published today by Nature, researchers describe broken bones of a mastodon (an extinct relative of elephants) and battered rocks from a site in southern California. The team argues that the remains demonstrate that humans were in the Americas 130,000 years ago, in the early late Pleistocene period. If they are right, the find could call into question the long-held assumption that Homo sapiens was the first and only member of the human family to reach the New World, because it hails from a time when multiple human species, including the Neandertals, roamed the planet. It could also suggest archaeologists have missed a more than 100,000-year record of humans in this part of the world. But the announcement has met with sharp criticism from other scientists, who variously argue that the remains do not necessarily reflect human activity, and that their age is uncertain.

Paleontologists excavated the remains in the early 1990s from a site in San Diego County, Calif., that was discovered during the course of highway improvements to State Route 54. The researchers recovered bones of a number of different Ice Age species from different stratigraphic levels in the site. For the new study, Steven Holen of the San Diego Natural History Museum and his colleagues focused on the partial skeleton of a male mastodon found in this location, dubbed the Cerutti Mastodon site for its discoverer, study co-author Richard Cerutti, also at the San Diego Natural History Museum. The mastodon’s limb bones bear evidence of distinctive breaks called spiral fractures that wind around the long axis of the bone. Such fractures typically occur when force is applied to fresh bone. The ends of some of the bones were also broken off, and several large, battered stone cobbles lay nearby. When the team experimentally broke bones from the carcasses of large modern-day mammals using hammerstones and anvils, the resulting damage resembled that seen on the bones and stone cobbles from site. Together, the pattern of damage evident on the bones and stones, and the proximity of the rocks to the bones, suggest to the team that humans were pounding the bones with the rocks to get to the nutritious marrow inside or to make bone tools.

None of that would be remarkable in and of itself. Such behaviors have been well documented at archaeological sites around the world. What makes the discovery a big deal is the supposed age of the remains. The team determined the age of the mastodon bones by applying a technique called uranium series dating, which uses the radioactive decay of uranium to measure the passage of time. The results indicated the bones are 130,000 years old, give or take 9,000 years—more than 100,000 years older than the oldest commonly accepted archaeological sites in the Americas.

Today the Cerutti Mastodon site sits in the middle of an urban setting. But 130,000 years ago, during the last interglacial period, it was a meandering stream in a flood plain near the coastline. Camels, dire wolves and capybara roamed there. “It was a very nice place to live,” Holen said at a press teleconference on April 25.

If Holen and his colleagues are correct about the age and nature of the finds, researchers will need to rethink everything they thought they knew about the peopling of the New World, including which human species was the first to colonize it. Most researchers agree that humans came to the Americas from northeastern Asia. At 130,000 years ago, the authors argue, Homo sapiens, Homo erectus, the Neandertals and the Denisovans (a group known only from ancient DNA recovered from Denisova cave in Siberia) might have been present in that part of the world. They could have crossed Beringia on foot prior to 135,000 years ago, when sea levels were sufficiently low. Otherwise, they could have traveled by boat, following the coasts of Asia, Beringia and North America to reach the latitude of the Cerutti Mastodon site.

During the press teleconference Holen said the new find should encourage other archaeologists to go out and look for more sites of this age—something he says they hadn’t done previously because no one expected humans to be in the Americas so early.

Experts not involved in the new study expressed deep skepticism about the team’s assessment, particularly the claim that the broken bones and battered stones reflect human activity. “You can’t push human antiquity in the New World back 100,000 years based on evidence as inherently ambiguous as broken bones and non-descript stones—not when they are coming from a highway salvage excavation done 25 years ago, and you have none of the detailed taphonomic evidence demanded of such a grandiose claim,” says David Meltzer of Southern Methodist University, an authority on the peopling of the Americas.

That lack of taphonomic evidence—information about what happened to the remains between when they were deposited and when they were discovered—comes down to “the difference between paleontological and archaeological excavation,” says archaeologist Andy Hemming of Florida Atlantic University, referring to the different approaches scientists use to unearth fossils as opposed to traces of material culture, which require more detailed provenience. “They didn’t map in every plottable object and pay attention to the relationships between items. Were pieces found 15 feet apart or 15 centimeters apart?” Such information is important for reconstructing how the bones broke and what, if any, relationship existed between the bones and the rocks.

Although the researchers were able to experimentally reproduce the damage on the remains by processing fresh bone with stone tools, critics observe, the team did not rule out alternative causes. “It is one thing to show that broken bones and modified rocks could have been produced by people, which Holen and his colleagues have done. It is quite another to show that people, and people alone, could have produced those modifications. This, Holen [and his colleagues] have most certainly not done, making this a very easy claim to dismiss,” says archaeologist Donald Grayson of the University of Washington. Other commenters explained that the team needs to look at many more fossil assemblages of large mammal bones, to see if natural causes could explain the breakage patterns evident in at the Cerutti Mastodon site.

Neither is simple hammerstone and anvil technology alone what many experts expect to see at a 130,000-year-old site. James Adovasio of Florida Atlantic University says that butchery sites of comparable age from other parts of the world tend to contain incontrovertible stone tools. He notes that by this time period humans were master stone knappers, capable of creating a variety of sophisticated, sharp-edged tools for cutting and slicing. “The utter absence of these things here is, shall we say, perplexing,” he comments. Adovasio led the excavations at the controversial site of Meadowcroft in Pennsylvania that dates to perhaps 16,000 years ago.

The possibility that archaic humans might have made it to the New World is another stumbling point for some critics. The Bering straits were flooded 130,000 years ago, notes Jon Erlandson of the University of Oregon, a lead proponent of the coastal route model of the peopling of the Americas.There’s some evidence that Homo erectus was able to cross a few small bodies of water, but no evidence that erectus, or Neandertals for that matter, could do long-range voyaging or that they had sophisticated boats like modern humans had when they colonized Australia.”

Species questions notwithstanding, if humans did enter the New World as early as Holen and his collaborators would have it, why is there such a yawning gap in the archaeological record between the Cerutti Mastodon remains and the next oldest sites in the Americas? “If there were people in San Diego 130,000 years ago, you have to explain why there weren’t any more of them there until 115,000 years after that,” Erlandson contends.

He takes issue with the authors’ suggestion that investigators simply have not been looking for remains that old, noting that he and other archaeologists have been doing exactly that for quite some time, often through the same sort of construction monitoring efforts that led to the discovery of the Cerutti Mastodon site. “I’ve done quite a bit of construction monitoring in the Santa Barbara area and we’ve carefully monitored excavations down to sediments of the same age. We were looking out for artifacts and didn’t find them,“ he says. “It boggles the mind that no one has found anything despite decades of geological monitoring.” Erlandson adds that there is a long history of people making claims for extraordinarily early sites in the Americas, including the site of Calico Hills in California, which the famed Kenyan paleoanthropologist Louis Leakey argued was perhaps 200,000 years old. But these claims have all been debunked.

Not only are there no other traces of humans in the Americas anywhere near 130,000 years old, there are also no any signs of human activity in the region from which humans are thought to have first entered the New World. “There is not a whisper of anything that age in northeast Asia,” observes archaeologist Robin Dennell of the University of Exeter in England, who studies the dispersal of human ancestors across Asia, Australia and the Americas.

For his part, Dennell is not bothered by the team’s interpretation of the bones and stones as signs of human activity. But he is concerned about the dating.The case for the site being 130,000 years old appears to rest on just three uranium-series dates,” he observes. “I’d want to see Cerutti Mastodon covered in more dates than a [date] palm tree before claiming it was in the last interglacial.”

Archaeological dating experts not involved in the research had mixed reactions to the study. “I think the dating is sound,” says geochronologist Rainer Grun of Griffith University in Queensland, Australia. But geochemist Bonnie Blackwell of Williams College thinks the team could do more to bolster its case. Bone is spongy and uranium can be absorbed into it or leeched out of it in ways that affect the accuracy of the dating results. She would like to see the mastodon teeth from the site dated using a technique called electron spin resonance (ESR) that looks at the electrons in the tooth enamel to estimate age. Blackwell has used a combination of uranium series and ESR to successfully date mastodon remains from the site of Hopwood Farm in Illinois.

“We need to leave our minds open. I admire these colleagues for sticking their necks out. They should be commended for doing that,” says Tom Dillehay of Vanderbilt University, who fought for years to convince the archaeological community that remains from the controversial site of Monte Verde in Chile pre-date the Clovis culture. Today most scholars accept that Monte Verde dates back to around 15,000 years old, if not 18,000 to 20,000 years ago, as Dillehay would have it. “But more evidence is going to be needed” for something this early, he says of the claims for human activity at the Cerutti Mastodon site.

Hemming agrees. “I’m all for hominins in the Americas by 130,000, but not on this evidence. There’s not enough to open the champagne.”

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